Frequency modulating system including feedback clamping circuit

ABSTRACT

A frequency modulating system for frequency-modulating an input signal with a predetermined carrier frequency comprising a controller circuit and a frequency modulator is provided. The controller circuit includes an automatic frequency detecting circuit, a voltage controlled oscillator, an error current generator, a feedback clamping circuit, a deviation current generator, and an adder circuit generating a frequency deviation/carrier frequency correction signal provided to the frequency modulator. The frequency modulator modulates the frequency in response to an output of the controller circuit and includes an oscillator having the same structure as that of the voltage controlled oscillator.

This is a division of application Ser. No. 08/160,873, filed Dec. 3,1993.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a frequency modulating circuit. Moreparticularly, it relates to a frequency modulating circuit which isautomatically controlled without an operator for controlling a carrierduring frequency modulation.

(2) Description of the Related Art

A video tape recording (VTR) system is capable of recording an audiosignal and video information for replay when desired. In this system,video signals are stored as signals having a modulated frequency inrecording media such as magnetic tapes.

When the video signals are stored on a magnetic tape, luminance signalsare stored on the magnetic tape after frequency modulation. A frequencymodulating circuit generally includes an oscillator formed of a resistorand a capacitor and a control system mounted outside to control thedesired carrier frequency necessary for proper frequency modulation. Thecarrier frequency is externally adjusted with the use of a variableresistor. This external resistor produces a voltage control of thefrequency of the oscillator. The variable resistor control andresistor/capacitor oscillator form a voltage controlled oscillator(VCO).

In general, a VCO is used with a phase detector that compares areference signal of a predetermined carrier frequency with an output ofthe VCO which would otherwise oscillate freely. A phase difference isdetected by the phase detector, which generates an error voltagecorresponding to this phase difference. The phase difference is fed backto the VCO to control the output of the VCO to the reference frequency.

The frequency modulating circuit of the VTR conventionally has a VCOwhich modulates an input signal which is to be recorded. The carrierfrequency of the frequency modulating circuit is controlled by using theerror voltage obtained by the control loop of the VCO.

Conventionally, the frequency of the carrier frequency is preciselycontrolled by the manual adjustment of an external variable resistor.However, this conventional precision control of the carrier frequencyhas the disadvantage of raising the costs of production of the VTR. Inaddition, the manual adjustment of the variable resistor increases theprobability of error in the control.

Further, there is the need for a circuit which generates a 3.4 MHzoscillating signal to be used as a reference signal. Also, the phasedetector may generate a large amount of error in the circuit if thecarrier frequency increases.

SUMMARY OF THE INVENTION

Therefore, the present invention provides an improved frequencymodulating circuit with reduced production cost and enhanced precision.The frequency modulating circuit is self adjusting without the need forexternal control, which eliminates the additional assembly steps,imprecision and costs associated with the conventional manual adjustmentmethod.

In order to achieve the above and other objects, the present inventiondiscloses a frequency modulating system for frequency-modulating aninput signal with a predetermined carrier frequency. The circuitincludes an automatic frequency detecting circuit for comparing thephase of a constant frequency reference signal, which is lower than thecarrier frequency, to the phase of a signal obtained by dividing thefrequency more than several times. The circuit also includes a VCO andan error current generator which converts an output error voltage fromthe frequency detector circuit into a current.

The circuit includes a feedback clamping circuit for clamping thereference signal. The signal to be frequency-modulated is input to thefeedback clamping circuit, as well as a reference voltage whichdetermines the clamping level. A deviation current generator, alsoincluded in the frequency modulation circuit, has a transconductancecircuit for amplifying and converting an output of the feedback clampingcircuit into a current which corresponds to the output. An adder circuitadds the error current and the deviation current, producing a frequencydeviation/carrier frequency correction control, which is then providedto a frequency modulator, which modulates the video signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The scope of the present invention will be better understood by theordinarily skilled person in the art from the detailed description ofthe invention with reference to the drawings, in which:

FIG. 1 is a block diagram of a frequency modulating system in accordancewith the present invention;

FIG. 2 is a detailed schematic diagram showing an example circuit of thevoltage controlled oscillator of FIG. 1;

FIGS. 3A to 3D are waveform charts for depicting the operation of thefrequency detector circuit of FIG. 1;

FIG. 4 is a detailed schematic diagram showing an example circuit of thefeedback clamping circuit in FIG. 1;

FIG. 5 shows an output waveform chart of the feedback clamping circuitof FIG. 4;

FIG. 6 is a detailed schematic diagram showing an example circuit of atransconductance amplifier;

FIG. 7 shows an output waveform chart of the present invention; and

FIG. 8 is a graph showing the relationship between an input signal andthe amount that the corresponding frequency is modulated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a block diagram for showing a frequency modulating circuit inaccordance with the present invention having a correction controlcircuit which automatically controls the carrier frequency of thefrequency modulating circuit for use in recording a video signal on amagnetic tape. It is to be understood that while the invention isdescribed with reference to recording video signals, the presentinvention is equally applicable to similar signals, e.g., audio signals.

A video signal is frequency-modulated by a frequency modulator 2 via afrequency correction controller 1. The frequency correction controller 1includes an error current generator 10 and a deviation current generator11. The output I_(err) from the error current generator 10 and theoutput I_(dev) from the deviation current generator 11 collect in adder12 and are output as a correction control signal I.

The error current generator 10 receives an externally provided referencesynchronous signal f_(H). The reference synchronous signal f_(H) may,for example, be derived from the video signal horizontal sync signal. Anerror current I_(err) is generated by a voltage-to-current converter104, which converts the error voltage V_(err) into the error currentI_(err). A phase-locked loop (PLL) is formed by an automatic frequencydetector 101, a frequency divider 102 and a voltage controlledoscillator 103.

The deviation current generator 11 generates a deviation current I_(dev)via a feedback clamping circuit 111 receiving the synchronous signalf_(H) and the video signal (video), a band gap reference circuit 112,and a transconductance amplifier 113 which operates as avoltage-to-current converter.

The PLL includes a VCO 103, which outputs a frequency signal in responseto the error voltage V_(err). The VCO of the preferred embodimentincludes oscillating transconductance amplifiers (OTA) as shown in FIG.2.

The VCO includes a first OTA 20 and a second OTA 21. An output V_(out)of the second OTA 21 is fed back to the inverting input of the first OTA20. A capacitor C₁ and an impedance circuit Z_(in) are coupled to theoutput of the first OTA 20 and are input to an input terminal of thesecond oscillating amplifier 21. A capacitor C₂ is coupled to the outputV_(out) of the second OTA 21. The VCO 103 outputs a frequencythree-hundred and twenty times that of the externally providedsynchronous signal f_(H).

The VCO includes an OTA filter formed from individual components, whichproduces an amount of freedom in the filtering of the signal by thechoice of components. Since the OTA filter can change thetransconductance and vary an oscillating frequency, it is suitable forthe free adjustment of the oscillating frequency by the selection ofcomponents having constant characteristics, such as a resistor. Aplurality of filters can be controlled collectively. The OTA filterforms an oscillator, and when the oscillation frequency changes forreasons such as thermal changes, the amount of variation in theoscillating frequency of the VCO 103 becomes similar to any changesoccuring in the frequency modulator 2.

The VCO 103 of the error current generator 10 has a frequency of, e.g.,5.04 MHz. The 5.04 MHz signal is input to the frequency divider circuit102 which divides the input signal by 320 to be similar to thesynchronous signal f_(H). The output of the 1/320 frequency divider isprovided to the frequency detecting circuit 101. The frequency detectingcircuit compares the divided signal output from the frequency dividercircuit 102 with the synchronous signal f_(H) supplied externally. Ifthe signals do not accord with each other, an error voltage V_(err) isgenerated in response to the phase difference. The error voltage V_(err)is input to the VCO 103 to automatically control the oscillation of theVCO 103 at 5.04 MHz. The error voltage V_(err) is also input to thevoltage-to-current converter 104, where it is converted and output as acorresponding amount of error current I_(err).

Accordingly, an oscillator having the same structure as the VCO 103 isincluded in the frequency modulator 2, and therefore oscillates exactlyas desired. That is, e.g., at a frequency of 3.4 MHz in accordance withthe system of the preferred embodiment. The carrier frequency of thefrequency modulator 2 may be automatically controlled from the outside.

It will be appreciated that factors which may cause a change infrequency of the VCO in the frequency modulator 2 (e.g. temperature),will cause a similar change in the VCO 103 of the frequency correctioncontroller 1. The phase lock loop of the frequency correction controller1 will generate a correction signal Verr which holds both VCOs to adesired frequency.

Referring now to the VCO 103 of FIG. 2, biquad filters formed of twoOTAs are configured as a band pass filter (BPF) so that the VCOoscillates by means of the negative feedback of the output V_(out) tothe inverting input of the first OTA 20 and the negative resistance Zin.A transconductance value gm of the VCO 103 in the frequency converterand a trans-conductance value gm of the BPF are formed in the samemanner, and a selectivity value Q is chosen to be large so that the VCOis stable. A propagation function of the VCO 103 is expressed as:##EQU1## where G is an equivalent conductance, ω_(o) is ##EQU2##

f_(o) is a value of an output current controlled by a voltage input andis variable according to the transconductance value, gm. The outputcurrent is formed to have the same structure as the output current ofthe filter circuit, and if the VCO is controlled at a predeterminedfrequency, ω_(o) of the frequency modulator is also controlled by theoutput current. That is, a current is supplied by I_(err), and thefrequency modulator 2 can be operated as discussed above.

The oscillating frequency of the VCO 103, which is 320 times that of thesynchronous signal f_(H), is input to the frequency divider circuit 102for division by 320 to a frequency of the synchronous signal f_(H). Thefrequency divider circuit 102 includes a counter formed of flip-flopcircuits to generate the output signal.

An automatic frequency detecting circuit 101 phase-compares the dividedsignal from the frequency divider circuit 102 with the synchronoussignal f_(H), which generates an error voltage V_(err) by the methoddiscussed below.

FIGS. 3A to 3D show wave forms relating to the operation of thefrequency detector 101. An H-pulse having a 60% duty cycle is formed asshown in FIG. 3B. A gate pulse for demodulating the PLL is formed asshown in FIG. 3C, simultaneously with the division of the output of theVCO. A demodulated output is obtained by gating the H-pulse with thegate pulse, as shown in FIG. 3D. An integration is performed by anintegrator, which then generates the error voltage V_(err).

The error voltage V_(err) controls the oscillation frequency of the VCO103. For example, if the initial oscillating frequency of the VCO 103 islower than the synchronous signal f_(H), then the period of the gatepulse of FIG. 3C is increased, which increases the upper portion of thegated signal shown in FIG. 3D. Accordingly, the output of the frequencydivider 102 becomes higher and raises the oscillating frequency of theVCO 103. Therefore, the upper portion of the demodulated output becomessimilar to the lower portion of the demodulated output. At this time thephase comparison of the signals are equal, and the PLL is considered`locked`. The error voltage V_(err) is converted to an error currentI_(err) by the voltage-to-current converter 104.

The frequency modulator 2 is structurally the same as the VCO 103, andcontrols the frequency of oscillation the same way as it does the VCO103. Thus, the VCO 103 can oscillate as desired.

Before the video signal is applied to the present circuit, the peakamplitude of the video signal is formed to be aligned at a constantelectrical potential, and the peak amplitude of the signal is againaligned to a constant electrical potential in a feedback clamp circuit(FBC) 111. The voltage-to-current converter 113 generates a deviationcurrent I_(dev) which is proportional to the peak aligned video signal.The deviation current I_(dev) is added to the error current I_(err) inthe adder circuit 12, which generates a correction control signal Iwhich controls the carrier frequency and the deviation in the frequencyof the frequency modulator. The peak-aligned level and a white level ofthe video signal are modulated respectively to 3.4 MHz and 4.4 MHz.

One detailed example of a circuit forming the feedback clamping circuit111 is depicted in FIG. 4, and an example of the transconductanceamplifier 113 is depicted in FIG. 6.

The feedback clamping circuit 11 clamps the peak amplitude of an inputvideo signal I_(n) to a predetermined voltage level. A synchronoussignal f_(H) is input to the feedback clamping circuit 111. TransistorsQ₃ and Q₄ are connected to the collector of transistor Q₂ which receivesthe synchronous signal f_(H) at it's base. A reference voltage V_(ref)is input as the voltage level at which the peak amplitude of the inputvideo signal I_(n) is to be clamped. The input video signal I_(n) isapplied to base of transistor Q₃ via a p-type metal oxide semiconductortransistor (PMOS) PMOS₄. The collector voltage V_(a) of transistor Q₄ ischanged in accordance with the potential of the peak-amplitude of thevideo signal which is applied to base of transistor Q₃. If the voltagelevel of the peak-amplitude of the video signal is lower than thereference input voltage V_(ref), then the current flowing via transistorQ₄ is increased, and the capacitor C₂ connected between the collector ofthe transistor Q₄ and ground (GND) discharges via transistor Q₄.Accordingly, the collector voltage V_(a) of transistor Q₄ drops andcurrent flows in the emitter of transistor Q₇. The voltage of the gateof transistor PMOS₄ rises, raising the voltage V_(x), and thus operatesto bring V_(x) to the same level as the reference voltage V_(ref).

On the contrary, if the voltage level of the peak-amplitude of the inputvideo signal I_(n) is higher than the reference voltage V_(ref), thecapacitor C₂ is charged and the collector voltage V_(a) of transistor Q₄rises, increasing the emitter current of transistor Q₇. Thus, thevoltage at V_(x) is lowered to become equal to the referencevoltage_(ref).

FIG. 5 is a wave form chart of the feedback clamping circuit 111 andillustrates an oscillating synchronous signal f_(H), the voltagereference V_(ref) which determines the level to which the input videosignal will be clamped, and a clamped video signal O₁₁₁.

An output of the band gap reference circuit 112 provides a bias to thecurrent sources of the feedback clamping circuit 111. An output of thefeedback clamping circuit 111 is input to a transconductance amplifier113 as shown in FIG. 1. An example of the transconductance amplifier 113is shown schematically in FIG. 6. A band gap reference circuit 112provides a stable bias source from temperature changes, and applies aconstant voltage to the base of transistor Q₁. A constant current flowsin transistor Q₁ so long as the external input I₄ applied to the emitterof transistor Q₁ does not change.

The clamped video signal O₁₁₁ output from the feedback clamping circuit111 is applied to a transistor PMOS₃, and a constant voltage I₃ isapplied to transistor PMOS₄. PMOS₃ is connected symmetrically withtransistor PMOS₄. When the clamped video signal O₁₁₁ is the same as theconstant voltage I₃, the output O₁₁₃ becomes zero.

This circuit generates a deviation current I_(dev) which is proportionalto the voltage level of the input video signal to control the frequencydeviation of the frequency modulator 2.

An error current I_(err) is added to the deviation current I_(dev) inthe adder circuit 12 and input to the frequency modulator 2. A generatedfrequency-modulated signal is shown in FIG. 7.

The frequency modulator has an oscillator having the same structure asthe VCO 103 of the error current generator 10. The error voltage V_(err)is applied equally to the frequency modulator 2 as well as to the VCO103 to obtain a carrier frequency which is controlled automatically.When the error current I_(err) and/or the deviation current I_(dev)change, a gm value of the OTA in the frequency modulator 2 also changes,and therefore the oscillating frequency changes.

FIG. 7 illustrates the frequency-modulated signals that are formed inaccordance with various frequencies of input video signals. Frequenciescorresponding to the level of video signals are illustrated. A dark clipregion which is the lowest level and a white clip region which is theuppermost level are shown.

Frequency levels corresponding to the levels of video signals are shownin the graph of FIG. 8. It is shown that, for example, a dark clip levelof 75% becomes 2.65 MHz at the time of frequency modulation.

A second oscillator for oscillating a frequency of 3.4 MHz is notnecessary by applying the OTA of the present invention. In addition,since the input frequency signal is compared to a synchronous signal, ahigh degree of accuracy in the detection of the phase to obtain acorrect error voltage V_(err) is conventionally difficult at thefrequency of the input signal. Therefore, the present invention providesfor a phase comparison to be carried out at a much lower frequency. Forinstance with the synchronous signal f_(H) at 15.625 kHz, makingpossible a more exact phase comparison.

Therefore, the present invention provides for a control in the degree ofaccuracy of the frequency modulation by including a circuit capable ofcontrolling small deviations in the frequency due, inter alia, to theinstabilities of components.

Although preferred embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood bythose skilled in the art that many variations and/or modifications ofthe basic inventive concepts herein taught are within the spirit andscope of the present invention, as defined in the appended claims.

What is claimed is:
 1. A frequency modulation system forfrequency-modulating an input signal with a predetermined carrierfrequency including a controller portion and a frequency modulatorportion, said controller portion comprising:voltage controlledoscillator means for generating a VCO frequency which is greater than areference frequency signal, said frequency varying in response to aninput error voltage signal; frequency divider means for dividing saidVCO frequency having a frequency divided output; automatic frequencydetecting means for comparing a phase of said reference frequency signalhaving a frequency lower than said carrier frequency with a phase ofsaid frequency divided output, said automatic frequency detecting meansgenerating said error voltage signal; and voltage-to-current convertermeans for converting said error voltage signal to an error currentsignal; an oscillator in said frequency modulator portion havingsubstantially similar structure as that of said voltage controlledoscillator means; feedback clamping means for clamping said referencefrequency signal and said input signal to be frequency-modulated inaccordance with a clamping level reference voltage, said feedbackclamping means including:a differential amplifying stage having a firstand a second transistor and having input said clamping level referencevoltage and said input signal to be frequency-modulated, a capacitor forcharging and discharging electricity coupled between a collector of saidsecond transistor and a ground, a third transistor coupled between saidground and a power source, said third transistor having an emittercurrent which increases and decreases in response to a variation in thevoltage across said capacitor, and a PMOS transistor having a gatereceiving said input signal and equalizing a source voltage andreference voltage in response to said third transistor; deviationcurrent generator means including a transconductance circuit foramplifying and converting an output of said feedback clamping means intoa current deviation signal; and adder means for combining said errorcurrent signal with said current deviation signal and for generating afrequency deviation/carrier frequency correction control signal; saidfrequency modulator portion modulating frequency in response to saidfrequency deviation/carrier frequency correction control signal.
 2. Afrequency modulation system for frequency-modulating an input signalwith a predetermined carrier frequency including a controller portionand a frequency modulator portion, said controller portion comprising:avoltage controlled oscillator generating a VCO frequency which isgreater than a reference frequency signal, said frequency varying inresponse to an input error voltage signal; a frequency divider dividingsaid VCO frequency having a frequency divided output; an automaticfrequency detector comparing a phase of said reference frequency signalhaving a frequency lower than said carrier frequency with a phase ofsaid frequency divided output, said automatic frequency detectorgenerating said error voltage signal; and a voltage-to-current converterconverting said error voltage signal to an error current signal; anoscillator in said frequency modulator portion having substantiallysimilar structure as that of said voltage controlled oscillator; afeedback clamping circuit clamping said reference frequency signal andsaid input signal to be frequency-modulated in accordance with aclamping level reference voltage, said feedback clamping circuitincluding:a differential amplifying stage having a first and a secondtransistor and having input said clamping level reference voltage andsaid input signal to be frequency-modulated, a capacitor for chargingand discharging electricity coupled between a collector of said secondtransistor and a ground, a third transistor coupled between said groundand a power source, said third transistor having an emitter currentwhich increases and decreases in response to a variation in the voltageacross said capacitor, and a PMOS transistor having a gate receivingsaid input signal and equalizing a source voltage and reference voltagein response to said third transistor; a deviation current generatorincluding a transconductance circuit for amplifying and converting anoutput of said feedback clamping circuit into a current deviationsignal; and an adder combining said error current signal with saidcurrent deviation signal and generating a frequency deviation/carrierfrequency correction control signal; said frequency modulator portionmodulating frequency in response to said frequency deviation/carrierfrequency correction control signal.
 3. A feedback clamping circuitclamping a reference frequency signal and an input signal to befrequency-modulated in accordance with a clamping level referencevoltage, said feedback clamping circuit including:a differentialamplifying stage having a first transistor and a second transistor andhaving input said clamping level reference voltage and said input signalto be frequency-modulated; a capacitor for charging and dischargingelectricity coupled between a collector of said second transistor and aground; a third transistor coupled between said ground and a powersource, said third transistor having an emitter current which increasesand decreases in response to a variation in voltage across saidcapacitor; and a transistor having a gate receiving said input signaland equalizing a source voltage and reference voltage in response tosaid emitter current of said third transistor.
 4. A feedback clampingcircuit clamping a reference frequency signal and an input signal to befrequency-modulated in accordance with a clamping level referencevoltage, said feedback clamping circuit including:differentialamplifying means including first transistor means and second transistormeans and having input said clamping level reference voltage and saidinput signal to be frequency-modulated; a capacitor for charging anddischarging electricity coupled between a collector of said secondtransistor means and a ground; third transistor means, coupled betweensaid ground and a power source, for providing a signal which increasesand decreases in response to a variation in voltage across saidcapacitor; and fourth transistor means, receiving said input signal, forequalizing a source voltage and reference voltage in response to saidsignal provided by said third transistor means.